Frame size is mostly about leg length (inseam) – the distance from the floor to your crotch when standing in socks. Too big and you can’t straddle the bike; too small and you’ll be cramped. Kids’ bikes are sized by wheel diameter.
How to use the Bike Size Calculator
- Bike for
- Bike type
- Rider height
- Inseam (0 = estimate from height)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose adult or child bike and the type, then enter your height and, if you know it, your inseam.
Inputs explained
| Field | What to enter |
|---|---|
| Bike for | Choose one: Adult, Child |
| Bike type | Choose one: Road / gravel, Mountain bike, City / hybrid / trekking |
| Rider height | Units: cm, m, ft, in |
| Inseam (0 = estimate from height) | Units: cm, m, ft, in |
Example values
How the main result changes when Rider height changes and everything else stays at the example values above:
| Rider height (cm) | Frame size (cm) |
|---|---|
| 87.5 | 28 |
| 131.25 | 41 |
| 175 | 55 |
| 218.75 | 69 |
| 262.5 | 83 |
| 350 | 110 |
Formula
Road: frame (cm) = inseam × 0.67 · Mountain: frame (in) ≈ inseam (cm) × 0.226 · City/hybrid: frame (cm) = inseam × 0.64 · Inseam ≈ 0.47 × height if unknown
Worked example
Inputs — Bike for: Adult; Bike type: Road / gravel; Rider height: 175 cm; Inseam (0 = estimate from height): 0 cm.
Results — Inseam used = 82.3 cm; Frame size = 55 cm; Frame size = 21.7 in; Typical label = L.
Key terms
- Inseam
- Inside leg length from floor to crotch.
Frequently asked questions
How do I measure my inseam?
Stand against a wall with a book between your legs and measure floor to the book's top.
What if I'm between sizes?
Pick the smaller frame for agility, larger for stability – and test ride.
Why are mountain bikes sized in inches?
Tradition; they also have shorter seat tubes for standover clearance.
Do brands size the same?
No – always check the brand's geometry chart.